The Zodiac and its division
Even though the Sun covers 30deg in the zodiac in one solar month that doesn’t form a basis and somebody might ask, why not take a solar month of20 days whereby Sun would cover 20deg in the zodiac with 18 zodiacal signs and 18 solar months. This looks very arbitrary unless we have a firm basis which cannot be questioned. We can arrive at that when we see the relative motion of two luminaries who guide our life the most, the Sun and the Moon. Both are equally important, what is Sun for the Day, we have Moon for the night. We know that Moon takes approximately 29.5 days after one conjunction with Sun to return back and conjoin it again. Thus we have the duration covered by Sun between two consecutive conjunctions of the luminaries alone approximate month. This corresponds to 12 sinodic month or soli-lunar month. When we add up the degrees covered by the 12 sinodic month, we shall fall short of 6 degrees, which if we distribute among the 12 months, make each month of 30 deg of solar motion.
Moon takes approximately 27.33 days to cover the whole zodiac of 360degree. If we find the duration covered by Sun, when Moon has covered one full circle of the zodiac, we will arrive at the approximate solar month of30 degrees solar motion.
THE ASCENDANT, DESCENDANT & MID HEAVEN
When the earth revolves around its axis, different signs appear in the sky, which rise from the eastern horizon and then set in the western one exactly like the Sun. At Sunrise, the sign where Sun is placed becomes the Ascendant as this sign rises with the rising of Sun. However as the day progresses and Sun moves from east to west, different sign keeps rising at the eastern horizon. At the moment of the birth, the sign which was rising at the eastern horizon becomes the ascendant and means something which has ascended from the below the horizon to above it.
The sign which is exactly above the head or meridian becomes the Mid-Heaven or the Xth house. Let’s take a case to visualise this. Let’s say for a native Sagittarius is rising at the Horizon when he took birth. This means that the signs Capricorn, Aquarius till some part of Gemini is below the horizon and will rise in that order. The sign which had risen just before the birth was Scorpio and a while ago it was Libra. If we distribute the day into four parts, each part being 6 hrs long, we can say that just before 6 hrs of birth Leo was rising. However, at birth, which happens to be 6 hrs from the point of time Leo was rising, it has moved exactly above our head. When we count zodiacally from Sagittarius to Leo, we see that Leo happens to fall in the 10th house from the Ascendant. Thus the sign which is exactly above the head (Zenith) during the birth becomes the MC/Mid-heaven/ Xth.
Similarly the sign which was setting in the western horizon during the birth is called is the Descendant as it is descending down the horizon. This is exactly opposite to the ascendant. Thus Gemini being exactly opposite to the Ascendant becomes the descendant to the native. This always falls in the 7th house from the Lagna or Ascendant.
The 4th house which would rise after 6 hrs of birth is in the Nadir as opposed to the Zenith. This happens to be at the lowest end of the horizon.
The signs from Descendant to the Ascendant which had already risen at the time of birth remains above the horizon and forms the visible half, whereas the signs from the Ascendant to Descendant which are yet to rise forms the Invisible half of the zodiac. The terms are quite self-explanatory as we can only see those signs which are above horizon and forms the visible half and those who are below the horizon remain invisible and form the invisible part of the zodiac.